Batch fermentation of an inoculated medium with Xanthomonas campestris NRRL B-1459 for 36-72 hours under aerobic conditions results in the formation of xanthan gum, which is separated from the other components of the medium by precipitation with acetone or methanol in a known manner. Because of time required to ferment each batch, the low biopolymer content of the fermented medium and the processing required for the recovery and purification of the product, xanthan gum produced by batch fermentation, hereinafter also referred to as xanthan, is relatively expensive.
Because continuous operation of a fermentation process offers a number of potential advantages over conventional batch methods that could be reflected in lower costs, considerable effort has been put forth in the past to perfect conditions that would support a reliable continuous process. But even with a continuous process a cheap medium from which xanthan can be produced is required. In addition to the necessity of an inexpensive medium in the manufacture of a low cost xanthan product, the ratio of xanthan to cells (bacteria) should be as high as possible in order to reduce subsequent filtration costs for cell removal. The specific productivity of the culture employed also should be as high as possible in order to maintain the aforesaid high ratio as well as to reduce vessel volume and capital costs. The expression "specific productivity" as used in the present description is intended to mean the number of grams of xanthan produced/gram of cells/hour. The culture should be stable under continuous culture conditions on a long term basis to avoid frequent restarts and lost productivity.
Although xanthan has been produced by continuous fermentation in the past, such methods have not met with unqualified success. In some cases, vitamins and/or amino acids had to be employed in the medium in substantial quantities in order to avoid culture degeneration or to improve specific productivity. Use of these additives, as well as soybean protein, cotton seed protein, etc., all tend to make the xanthan thus produced more costly.
It is well known that the continuous production of xanthan by the use of Xanthomonas campestris B-1459 has been hampered by a tendency of the culture to change or degenerate after a fairly small and specific number of turnovers, the time during the fermentation to completely replace one volume of broth in the fermentation vessel. Normally, 6-9 turnovers are the maximum that can be obtained before degeneration of the culture occurs. At the same time, there is a decrease in viscosity, a loss in volumetric productivity of xanthan, i.e., grams of xanthan/liter of broth/hour, and appearance of a variety of culture variants or strains that no longer produce xanthan or else produce a xanthan of low quality. It has been demonstrated that culture degeneration occurs when dried distillers solubles (DDS) are used in the nutrient medium as the complex nitrogen source, whether in the whole form or as a water soluble extract. In other cases, certain strains of Xanthomonas have been grown successfully without culture degeneration in simple minimal media, but the xanthan:cell ratio and specific productivity have been low, on the order of 0.1-0.12 gram xanthan/gram of cells/hour.
Earlier work has indicated that heteropolysaccharides produced by the action of Xanthomonas bacteria on carbohydrate media have potential applications as film forming agents, as thickeners for body building agents in edible products, cosmetic preparations, pharmaceutical vehicles, oil field drilling fluids, fracturing liquids and similar compositions and as emulsifying, stabilizing and sizing agents. Heteropolysaccharides, particularly xanthan gum, have significant potential as a mobility control agent in micellar polymer flooding. This gum has excellent viscosifying properties at low concentration, is resistant to shear degradation and exhibits only minimal losses in viscosity as a function of temperature, pH and ionic strength. For these reasons, xanthan gum is an attractive alternative to synthetic polyacrylamides for enhanced oil recovery operations.